Method of crosscutting a moving web

ABSTRACT

A method of cross-cutting a moving web uses a cutting knife operably mounted on a knife cylinder having a circumferential distance L Z  where L Z  is smaller than a cut length, L A . The knife cylinder for revolving in a first and a second opposite direction is powered by a drive motor in either direction. The method includes revolving the knife cylinder in a first direction, cutting the web with the cutting knife, and completing a first cut. The knife cylinder is then reversed to retard progress in the first direction and then accelerated to the speed of the moving web until starting the second cut.

BACKGROUND OF THE INVENTION

[0001] 1. Priority Claim

[0002] This application claims priority to application DE 102 13 978.4 filed Mar. 28, 2002 in Germany.

[0003] 2. Field of the Invention

[0004] The invention relates to a device for and a method of cross-cutting a moving web using a knife cylinder having at least one cutting knife which rotates about an axis parallel to the cutting line.

[0005] 3. Description of the Related Art

[0006] DE 100 30 055 Al discloses a method of cross-cutting a web using a knife cylinder. In order to vary the length of the cut products, i.e. vary the cut length, other than during cutting, the knife cylinder is driven at a circumferential speed that differs from the speed that the web is advanced. When small cut lengths or when the knife cylinder has a plurality of cutting knives distributed uniformly over the circumference, the speed change has to be completed during a small amount of circumferential travel. This requires high accelerations and powerful up-shifting, i.e. acceleration torque, of the drive motor. Consequently, this presents practical limitations when implemented.

SUMMARY OF THE INVENTION

[0007] It is an object of the invention to provide a cross-cutting method in which the required acceleration torque of the knife cylinder is low.

[0008] According to the invention, this and other objects are achieved by a method for cross-cutting a moving web with a cutting knife operably mounted on a knife cylinder having a circumferential distance L_(z) where L_(z) is smaller than a cut length L_(A). The knife cylinder for revolving in a first and second opposite direction is powered by a drive motor in either direction. The method includes revolving the knife cylinder in a first direction, cutting the web with the cutting knife, and completing a first cut. The knife cylinder is then reversed to retard progress in the first direction and then accelerated to the speed of the moving web until starting the second cut with the cutting knife.

[0009] In order to achieve a low acceleration torque of the knife cylinder with high speed changes, a low moment of inertia is striven for. This is advantageously achieved by using a knife cylinder with a small diameter. Therefore, often the circumference of the knife cylinder is small in comparison with the cut length. Thus, the circumference over which the knife cylinder can be accelerated for the subsequent cut is correspondingly reduced. However, by reversing the travel direction of the knife cylinder following the first cut, the length over which a speed change may occur is lengthened; in fact, it is configured so as to overlap. Overlapping the lengths over which a speed change in the cylinder may occur when cut lengths are greater than the circumference of the knife cylinder has the advantage that the maximum driven moment of the motor needed to change the speed decreases. This occurs because the braking and acceleration travel based on the circumference of the knife cylinder may be increased up to twice the circumferential value, because of the overlap.

[0010] Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a view of an apparatus for cross-cutting a web.

[0012]FIG. 2 shows a view of the course of the necessary drive torque for a knife cylinder fitted with one cutting knife.

[0013]FIG. 3 shows a view of the rotary angle of the knife cylinder with respect to the web position based on a cut length.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0014] The apparatus for cross-cutting a web 1, shown in FIG. 1, includes a knife cylinder 2 fitted with at least one cutting knife 3 which during the rotation of the knife cylinder 2 rotates about its axis of rotation, wherein the axis of rotation is parallel to the cutting line. The axis of rotation is indicated by the curved double-headed arrow. Therein, the axis is perpendicular to the plane of FIG. 1.

[0015] The cutting knife 3 cooperates with an opposing knife 4 which is arranged in a fixed position. While the opposing knife 4 may rotate about an axis, it may also be designed as a cutting bar. Such a cutting bar may, for example, be housed in a folding cylinder, for example as a folding-blade cylinder of a folder. The cutting knife 3 and the opposing knife 4 can advantageously be arranged with an angular offset in relation to the cutting line to be made. This may advantageously produce a shearing cut.

[0016] It is also possible for a plurality of cutting knives to be arranged distributed uniformly on the circumference of the knife cylinder 2. For example, a second cutting knife has been indicated on cylinder 2 in FIG. 1 by thinner lines and labelled as (3), i.e. Item 3 is placed in brackets. The knife cylinder 2 is driven by a motor 5 in the form of an electric motor.

[0017] During a complete revolution, the knife cylinder 2 cuts a sheet 6 from the web 1 as the cutting knife 3 passes the opposing blade 4. This sheet may also be a signature or a product.

[0018] During the cut, the circumferential speed of the cutting knife 3 is approximately equal to the speed of the web 1 where web 1 advances substantially in the direction of the single headed arrow.

[0019] In accordance with one embodiment of the invention, when a single cutting knife 3 is disposed on a knife cylinder 2 with a circumference L_(z). and the sheet has the cut length L_(A), the cut area is defined by the product of L_(z) and L_(A). In accordance with one embodiment of the invention, when two or more cutting knives 3 are disposed on the knife cylinder 2, L_(z) expresses the circumferential spacing between two cutting knives 3 cutting one after another, wherein the circumference L_(z) has a smaller dimension than the cut length L_(A).

[0020] In one embodiment, the ratio between L_(A) and L_(z) is advantageously chosen to be in the region B2 of the bar element in FIG. 2. In this region B2, the knife cylinder 2 is advantageously operated with direction reversal. Therein, after the cut, the knife cylinder 2 is retarded with direction reversal and is then accelerated to the web speed until the next step. As a result, the maximum drive torque M of the motor 5 needed to change the circumferential speed decreases, since the braking and acceleration travel based on the circumference L_(z) of the knife cylinder 2 may be increased as far as approximately twice the circumferential value, because of the overlap. The drive torque M then assumes the course illustrated by dashed line in FIG. 2 wherein knife cylinder 2 fitted with one cutting knife 3.

[0021] When the maximum drive torque M2 is known, i.e. is predefined, the region B2 can be used to dimension the drive motor. Without direction reversal, the result would be the limiting value M1 for the maximum drive torque.

[0022] The advantage of the torque reduction as a result of speed reversal is useful if the ratio to be cut between cut length L_(A) and circumference L_(z) of the knife cylinder 2 lies in the area b of FIG. 2. If in addition the operating point G1, at which the knife cylinder 2 comes to its brief stop during the speed change, is saved for operation, the drive torque can be restricted to the limiting value M2. A further region B1 for the operation of the device with a maximum drive torque M2 is indicated in FIG. 2.

[0023] When direction reversal is used, the direction of rotation of the knife cylinder 3 is reversed in a restricted region of one revolution or in a region between two cutting knives 3 cutting one after another in the case of the multiple arrangement of cutting knives 3 on the knife cylinder 2. The method uses the current position and speed of the web 1 and assigns to this the rotational angle Φ and the angular speed of the knife cylinder 2. In the area of the overlap between the braking and acceleration travels, one point on the circumference of the knife cylinder 2 has, for a restricted time period, an opposite sign to the direction of movement of the web 1.

[0024] For the purpose of technical implementation of the (angular) positions or (angular) speeds of web 1 and knife cylinder 2, the following criteria are met by the method illustrated in order to produce the algorithm:

[0025] continuity of the angular position and of the first derivative of the angular position of the knife cylinder 2;

[0026] preventing travel past the cut position during the phase of the speed change;

[0027] region of coincident speed between a point on the circumference of the knife cylinder and the web 1 during the cut and in a definable region before and after the time of the cut, where this region before and after the cut does not necessarily have to be of equal size; and

[0028] establishing that the above criteria have been met (stability of the algorithm) with the aid of characteristic numbers.

[0029] For the calculation of the compensation movement, a polynomial of third order (cubic splines), for example, has been determined to be suitable. In accordance with such a polynomial, the rotational angle Φ of the knife cylinder 2 is presented against the web position s, based on the cut length L_(A), in FIG. 3. A computing and storage unit 7 is connected to a motor controller 8 of the motor 5 of the knife cylinder 2 and forwards the control commands that implement this function, coordinated with the position of the printed image of the web 1, to the motor controller 8 (FIG. 1).

[0030] List of Reference Symbols:

[0031]1. Web

[0032]2. Knife cylinder

[0033]3. Cutting knife

[0034]4. Opposing knife

[0035]5. Motor

[0036]6. Sheet

[0037]7. Computing and storage unit

[0038]8. Motor controller

[0039] L_(A) Cut length

[0040] L_(z) Circumference of knife cylinder

[0041] B1 Region

[0042] B2 Region

[0043] M Drive torque

[0044] M1 Max. drive torque without direction reversal

[0045] M2 Max. drive torque with direction reversal

[0046] a Area

[0047] b Area

[0048] s Web position

[0049] G1 Operating point

[0050] Φ Rotational angle

[0051] Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 

What is claimed is:
 1. A method of cross-cutting a moving web (1) with a cutting knife (3) operably mounted on a knife cylinder (2), wherein the knife cylinder has a circumferential distance L_(Z), L_(Z) being smaller than a cut length L_(A), the knife cylinder (2) for revolving in a first and a second opposite direction powered by a drive motor (5) in either direction; the method comprising the steps of: revolving the knife cylinder (2) in a first direction; cutting the web (1) with the cutting knife (3); completing a first cut; reversing the knife cylinder (2) to retard progress in the first direction; and accelerating the knife cylinder (2) to the speed of the moving web (1) until starting a second cut with the cutting knife (3).
 2. The method of claim 1 further comprising the first and second steps, respectively, of: predetermining a location on the moving web; and selecting a cubic spline polynomial so that the second cut is made in a predetermined location on the moving web.
 3. The method of claim 1 further comprising the first and second steps, respectively, of: predetermining a maximum torque M2 of the drive motor, selecting cut length L_(A) to so that torque M2 is achieved at the end of the step of accelerating the knife cylinder to the speed the moving web until starting the second cut.
 4. A method of cross-cutting a moving web (1) with a first cutting knife (3) and a second cutting knife operably mounted on a knife cylinder (2), wherein the first and second cutting knives are separated by a circumferential distance L_(Z), L_(Z) being smaller than a cut length L_(A), the knife cylinder (2) for revolving in a first and a second opposite direction powered by a drive motor (5) in either direction; the method comprising the steps of: revolving the knife cylinder (2) in a first direction; cutting the web (1) with the first cutting knife (3); completing a first cut; reversing the knife cylinder (2) to retard progress in the first direction; and accelerating the knife cylinder (2) to the speed of the moving web (1) until starting a second cut with the second cutting knife.
 5. The method of claim 4 further comprising the first and second steps, respectively, of: predetermining a location on the moving web; and selecting a cubic spline polynomial so that the second cut is made in a predetermined location on the moving web.
 6. The method of claim 4 further comprising the first and second steps, respectively, of: predetermining a maximum torque M2 of the drive motor, selecting cut length L_(A) to so that torque M2 is achieved at the end of the step of accelerating the knife cylinder to the speed the moving web until starting the second cut.
 7. A method of crosscutting a moving web (1) in a web-fed rotary press, having a knife cylinder (2) with at least one cutting knife (3) that rotates about an axis parallel to the cutting line, comprising the steps of: moving the web (1) with an approximately constant speed; rotating the knife cylinder (2) during a first cut at a circumferential speed that corresponds approximately to the web speed; rotating the knife cylinder (2) otherwise at a changing circumferential speed, the circumference of the knife cylinder (2) being different than the cut length (L_(A)).
 8. The method of claim 7 further comprising the steps of: retarding the knife cylinder (2) with direction reversal after the first cut; accelerating the knife cylinder (2) to the web speed until a second cut, the circumference of the knife cylinder (2) is smaller than the cut length (L_(A)).
 9. The method of claim 7, wherein the knife cylinder (3) includes a second cutting knife, and the method further comprising the steps of: retarding the knife cylinder (2) with direction reversal after the first cut; accelerating the knife cylinder (2) to the web speed until a second cut, the circumferential spacing (L_(Z)) of the knife cylinder (2) between first and second cutting knife (3) cutting one after another is smaller than the cut length (L_(A)).
 10. The method of claim 7 wherein the step of rotating the knife cylinder (2) during a first cut at a circumferential speed that corresponds approximately to the web speed further comprises rotating the knife cylinder (2) in accordance with a function.
 11. The method of claim 7 wherein the step of rotating the knife cylinder (2) during a first cut at a circumferential speed that corresponds approximately to the web speed further comprises the step of rotating the knife cylinder (2) during the cut such that a position of the knife cylinder is synchronized with the web (1).
 9. The method of claim 4, further comprising the first step of: selecting a rate of rotation of the knife cylinder so that a variable cut length (L_(A)) corresponds with the permissible drive torque (M2) of a drive motor (5) and the circumferential spacing (L_(Z)) of two cutting knives (3) cutting one after another on the knife cylinder (2). 